Implement Graphviz, Figlet and Cowsay as Remarkup interpreter blocks
Summary: Fixes T3274. Fixes T3964. Ref T3591.
Test Plan: {F70928}
Reviewers: btrahan
Reviewed By: btrahan
CC: chad, aran
Maniphest Tasks: T3274, T3964, T3591
Differential Revision: https://secure.phabricator.com/D7332